Airsoft is quite expensive in Canada because of the restriction against importation. Prices are usually 2-3x what they are in the US. A quality, actually playable P90 will run from $400-$600.
If you really wanted a P90 only for Stargate reenactment *cough* larping/cosplay, then you could look for a clear plastic P90 springer which shouldn't be too expensive and I've seen them in some stores in Canada. You could spraypaint it black and maybe buy a P90 lowcap with visible bullets ($20) and install it permantely. This gun sucks though and it's only good as a prop. It wouldn't be useful to play it in an actual airsoft game but it's compentent for using as a prop.
The other gear for SG-1 is easy and cheap to get. You can just get a cheap Chinese clone black tactical vest and an olive drab BDU.